how to add up 2 points on EC operation about elliptic-php HOT 1 CLOSED

use Elliptic\EC;
use BN\BN;

$ec = new EC('secp256k1');

// See: http://bip32.org using Derive From BIP32 Key
// xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqsefD265TMg7usUDFdp6W1EGMcet8
$c_par = "873dff81c02f525623fd1fe5167eac3a55a049de3d314bb42ee227ffed37d508";
$K_par = "0339a36013301597daef41fbe593a02cc513d0b55527ec2df1050e2e8ff49c85c2";

// Derived public child key 
// Derivation path Simple: m/i 
// Keypair index i: 2018
// xpub68Gmy5EVb2Begkah8BxugKchT5SExW5p9gEHBLnEvYSuwVppt2TzD3WTjxNk14R8pmHbz3MHB9n75M2zNYgkJUCwV9pYwU9Z21Awj7Cr5U9
$expected_c_child = "a7470737ffde1458292e19e838534f400ad3c0f72e12f08eff79dee4fce11bed";
$expected_K_child = "0376499d06f9e9df71d7ee08d13a91337fa2b92182d4afcddf917b8d9983eb4615";

$i = 2018;
$I_key  = hex2bin($c_par);
$I_data = hex2bin($K_par) . pack("N", $i);
$I = hash_hmac("sha512", $I_data, $I_key);
$I_L = substr($I, 0, 64);
$I_R = substr($I, 64, 64);
$c_i = $I_R;

$K_par_point = $ec->curve->decodePoint($K_par, "hex");
$I_L_point = $ec->g->mul(new BN($I_L, 16));
$K_i = $K_par_point->add($I_L_point);
$K_i = $K_i->encodeCompressed("hex");

if ($expected_c_child == $c_i && $expected_K_child == $K_i) {
    echo "Success!\n";
} else {
    echo "Failure!\n";
}